'Nog' means 'still' in the sense that there not everything has been put away.
'Nog steeds' means 'still' in the time-sense. They were there a time ago and they are still there.

Ik heb hem nog niet gezien => Is Peter here? I haven't seen him yet.
Ik heb hem nog steeds niet gezien => I still haven't seen him. Meaning that you have been looking for him for a while, but still haven't seen him.